add transform function customization, remove hints

include/SDL3/SDL_hints.h

@@ -2505,35 +2505,6 @@ extern "C" {
  */
 #define SDL_HINT_MOUSE_RELATIVE_MODE_WARP "SDL_MOUSE_RELATIVE_MODE_WARP"
 
-/**
- * A variable setting the scale for mouse motion, in floating point, when the
- * mouse is in relative mode.
- *
- * This hint can be set anytime.
- *
- * \since This hint is available since SDL 3.1.3.
- */
-#define SDL_HINT_MOUSE_RELATIVE_SPEED_SCALE "SDL_MOUSE_RELATIVE_SPEED_SCALE"
-
-/**
- * A variable controlling whether the system mouse acceleration curve is used
- * for relative mouse motion.
- *
- * The variable can be set to the following values:
- *
- * - "0": Relative mouse motion will be unscaled. (default)
- * - "1": Relative mouse motion will be scaled using the system mouse
- *   acceleration curve.
- *
- * If SDL_HINT_MOUSE_RELATIVE_SPEED_SCALE is set, that will override the
- * system speed scale.
- *
- * This hint can be set anytime.
- *
- * \since This hint is available since SDL 3.1.3.
- */
-#define SDL_HINT_MOUSE_RELATIVE_SYSTEM_SCALE "SDL_MOUSE_RELATIVE_SYSTEM_SCALE"
-
 /**
  * A variable controlling whether a motion event should be generated for mouse
  * warping in relative mode.

include/SDL3/SDL_mouse.h

@@ -190,6 +190,37 @@ extern SDL_DECLSPEC const char * SDLCALL SDL_GetMouseNameForID(SDL_MouseID insta
  */
 extern SDL_DECLSPEC SDL_Window * SDLCALL SDL_GetMouseFocus(void);
 
+/**
+ * Retieve an implementation of the function by which the platform
+ * transforms raw mouse inputs into cursor motion.
+ *
+ * This returns a function pointer to the implmentation if one exists,
+ * or NULL if not. This pointer can be passed to SDL_SetInputTransform
+ * if one wishes to scale relative mouse motion using the implementation,
+ * in which case the userdata argument is ignored and platform-specific
+ * data will be used instead.
+ *
+ * \returns a pointer to an input transform function.
+ *
+ * \since This function is available since SDL 3.1.3.
+ *
+ * \sa SDL_SetInputTransform
+ */
+extern SDL_DECLSPEC const void * SDLCALL SDL_GetSystemInputTransform(void);
+
+/**
+ * Set a user-defined function by which to transform relative mouse inputs,
+ * and its associated internal data which will be passed to the function.
+ *
+ * \param transform a pointer to an input transform function, or NULL to disable.
+ * \param userdata an optional pointer to the associated internal data.
+ *
+ * \since This function is available since SDL 3.1.3.
+ *
+ * \sa SDL_GetSystemInputTransform
+ */
+extern SDL_DECLSPEC void SDLCALL SDL_SetInputTransform(const void *transform, void *userdata);
+
 /**
  * Query SDL's cache for the synchronous mouse button state and the
  * window-relative SDL-cursor position.

src/events/SDL_mouse.c

@@ -100,26 +100,6 @@ static void SDLCALL SDL_MouseNormalSpeedScaleChanged(void *userdata, const char
     }
 }
 
-static void SDLCALL SDL_MouseRelativeSpeedScaleChanged(void *userdata, const char *name, const char *oldValue, const char *hint)
-{
-    SDL_Mouse *mouse = (SDL_Mouse *)userdata;
-
-    if (hint && *hint) {
-        mouse->enable_relative_speed_scale = true;
-        mouse->relative_speed_scale = (float)SDL_atof(hint);
-    } else {
-        mouse->enable_relative_speed_scale = false;
-        mouse->relative_speed_scale = 1.0f;
-    }
-}
-
-static void SDLCALL SDL_MouseRelativeSystemScaleChanged(void *userdata, const char *name, const char *oldValue, const char *hint)
-{
-    SDL_Mouse *mouse = (SDL_Mouse *)userdata;
-
-    mouse->enable_relative_system_scale = SDL_GetStringBoolean(hint, false);
-}
-
 static void SDLCALL SDL_MouseWarpEmulationChanged(void *userdata, const char *name, const char *oldValue, const char *hint)
 {
     SDL_Mouse *mouse = (SDL_Mouse *)userdata;
@@ -220,12 +200,6 @@ bool SDL_PreInitMouse(void)
     SDL_AddHintCallback(SDL_HINT_MOUSE_NORMAL_SPEED_SCALE,
                         SDL_MouseNormalSpeedScaleChanged, mouse);
 
-    SDL_AddHintCallback(SDL_HINT_MOUSE_RELATIVE_SPEED_SCALE,
-                        SDL_MouseRelativeSpeedScaleChanged, mouse);
-
-    SDL_AddHintCallback(SDL_HINT_MOUSE_RELATIVE_SYSTEM_SCALE,
-                        SDL_MouseRelativeSystemScaleChanged, mouse);
-
     SDL_AddHintCallback(SDL_HINT_MOUSE_EMULATE_WARP_WITH_RELATIVE,
                         SDL_MouseWarpEmulationChanged, mouse);
 
@@ -696,13 +670,12 @@ static void SDL_PrivateSendMouseMotion(Uint64 timestamp, SDL_Window *window, SDL
 
     if (relative) {
         if (mouse->relative_mode) {
-            if (mouse->enable_relative_speed_scale) {
-                x *= mouse->relative_speed_scale;
-                y *= mouse->relative_speed_scale;
-            } else if (mouse->enable_relative_system_scale) {
-                if (mouse->ApplySystemScale) {
-                    mouse->ApplySystemScale(mouse->system_scale_data, timestamp, window, mouseID, &x, &y);
+            if (mouse->InputTransform) {
+                void *data = mouse->input_transform_data;
+                if (mouse->InputTransform == mouse->ApplySystemScale) {
+                    data = mouse->system_scale_data;
                 }
+                mouse->InputTransform(data, timestamp, window, mouseID, &x, &y);
             }
         } else {
             if (mouse->enable_normal_speed_scale) {
@@ -1049,12 +1022,6 @@ void SDL_QuitMouse(void)
     SDL_RemoveHintCallback(SDL_HINT_MOUSE_NORMAL_SPEED_SCALE,
                         SDL_MouseNormalSpeedScaleChanged, mouse);
 
-    SDL_RemoveHintCallback(SDL_HINT_MOUSE_RELATIVE_SPEED_SCALE,
-                        SDL_MouseRelativeSpeedScaleChanged, mouse);
-
-    SDL_RemoveHintCallback(SDL_HINT_MOUSE_RELATIVE_SYSTEM_SCALE,
-                        SDL_MouseRelativeSystemScaleChanged, mouse);
-
     SDL_RemoveHintCallback(SDL_HINT_MOUSE_EMULATE_WARP_WITH_RELATIVE,
                         SDL_MouseWarpEmulationChanged, mouse);
 
@@ -1083,6 +1050,17 @@ void SDL_QuitMouse(void)
     SDL_mice = NULL;
 }
 
+const void *SDL_GetSystemInputTransform(void) {
+    SDL_Mouse *mouse = SDL_GetMouse();
+    return mouse->ApplySystemScale;
+}
+
+void SDL_SetInputTransform(const void *transform, void *userdata) {
+    SDL_Mouse *mouse = SDL_GetMouse();
+    mouse->InputTransform = (SDL_InputTransform)transform;
+    mouse->input_transform_data = userdata;
+}
+
 SDL_MouseButtonFlags SDL_GetMouseState(float *x, float *y)
 {
     SDL_Mouse *mouse = SDL_GetMouse();

src/events/SDL_mouse_c.h

@@ -50,6 +50,8 @@ typedef struct
     Uint8 click_count;
 } SDL_MouseClickState;
 
+typedef void (*SDL_InputTransform)(void *userdata, Uint64 timestamp, SDL_Window *window, SDL_MouseID mouseID, float *x, float *y);
+
 typedef struct
 {
     // Create a cursor from a surface
@@ -83,9 +85,13 @@ typedef struct
     SDL_MouseButtonFlags (*GetGlobalMouseState)(float *x, float *y);
 
     // Platform-specific system mouse transform
-    void (*ApplySystemScale)(void *internal, Uint64 timestamp, SDL_Window *window, SDL_MouseID mouseID, float *x, float *y);
+    SDL_InputTransform ApplySystemScale;
     void *system_scale_data;
 
+    // User-defined mouse input transform
+    SDL_InputTransform InputTransform;
+    void *input_transform_data;
+
     // Data common to all mice
     SDL_Window *focus;
     float x;
@@ -105,9 +111,6 @@ typedef struct
     int relative_mode_clip_interval;
     bool enable_normal_speed_scale;
     float normal_speed_scale;
-    bool enable_relative_speed_scale;
-    float relative_speed_scale;
-    bool enable_relative_system_scale;
     Uint32 double_click_time;
     int double_click_radius;
     bool touch_mouse_events;

src/video/windows/SDL_windowsmouse.c

@@ -694,9 +694,9 @@ void WIN_UpdateMouseSystemScale(void)
 {
     SDL_Mouse *mouse = SDL_GetMouse();
 
-    if (mouse->ApplySystemScale == WIN_ApplySystemScale) {
-        mouse->system_scale_data = &WIN_system_scale_data;
-    }
+    // always set to platform impl to be safe, even though it's not exposed to user.
+    mouse->ApplySystemScale = WIN_ApplySystemScale;
+    mouse->system_scale_data = &WIN_system_scale_data;
 
     // always reinitialize to valid defaults, whether fetch was successful or not.
     WIN_MouseData *data = &WIN_system_scale_data;